Silver-based powder, method of preparation thereof, and curable silicone composition

ABSTRACT

A silver-based powder is surface-treated with an oxidation inhibitor by means of a mechanochemical reaction. A method of preparation of the silver-based powder includes using an organic solution of the oxidation inhibitor as a lubricating agent, applying mechanical energy to the silver-based powder, and subjecting the silver-based powder to surface treatment with said oxidation inhibitor by means of a mechanochemical reaction.

TECHNICAL FIELD

This invention relates to a silver-based powder, a method of preparationof the silver-based powder, and a curable silicone composition. Morespecifically, this invention relates to a silver-based powder thatprevents decrease of curability of a curable organic resin compositionin storage when the silver-based powder is compounded with the curableorganic resin composition. This invention further relates to a methodfor preparation the silver-based powder with high efficiency, and acurable silicone composition characterized by a reduced change incurability over time during storage and by an ability to be cured into acured silicone product with electrical properties that have minimalchanges over time.

BACKGROUND ART

It has been shown in Japanese Official Patent Publications (hereinafterreferred to as Kokoku) No. S62-53033, No. S62-53034, Japanese Laid-OpenPatent Application Publications (hereinafter referred to as Kokai) No.S58-103565, No. S58-103566, Kokoku No. S62-53035, and Kokai S58-104970that coating of the surface of a silver-alloy powder with benzotriazoleimproves migration-resistant properties. However, when the silver powdersurface-coated with benzotriazole or a commonly available silver powderis compounded with a curable organic resin composition, the powderimpairs curability of the composition in storage.

Silver powders are used with curable silicone compositions aselectrically conductive and thermally conductive fillers that improveelectrical and thermal conductivity of the compositions. However, thesurfaces of commonly available silver powders normally contain someresidual lubricants used in the manufacturing process, such as higherfatty acids, metal soaps, higher aliphatic alcohols or their esters,higher aliphatic amines, higher aliphatic amides, polyethylene waxes,etc. The presence of these residuals on the surfaces of the fillerssignificantly impairs curability of the curable silicone compositionsduring storage and some time later may lead to complete loss ofcurability.

Kokai No. H7-109501, Kokai No. H7-150048, and Kokai No. H8-302196contain some proposals aimed at a solution of problems associated withthe use of a silver powder surface-treated with an organosiliconcompound and of a curable silicone composition compounded with a silverpowder. However, the aforementioned proposals still appeared to beinsufficient for restricting the decrease of curability in storage ofthe aforementioned curable silicone compositions.

It is an object of this invention to provide: a silver-based powderthat, when compounded with a curable organic resin composition, does notdecrease curability of the composition in storage; a method forpreparation of the silver-based powder with high efficiency; and acurable silicone composition that is characterized by reduced changes incurability over time during storage and by an ability to be cured into acured product with electrical properties that have minimal changes overtime.

DISCLOSURE OF INVENTION

A silver-based powder of this invention is characterized by beingsurface-treated with an oxidation inhibitor by means of amechanochemical reaction. A method of this invention for preparation ofthe silver-based powder is characterized by utilizing an organicsolution of an oxidation inhibitor as a lubricating agent, applyingmechanical energy to the silver-based powder, and subjecting thesilver-based powder to surface treatment with the oxidation inhibitor. Acurable silicone composition of this invention comprises thesilver-based powder surface-treated with the oxidation inhibitor.

DETAILED DESCRIPTION OF THE INVENTION

The silver-based powder of this invention is characterized by the factthat it is surface-treated with an oxidation inhibitor by means of amechanochemical reaction. More specifically, the surface of thesilver-based powder can be activated and a chemical reaction with theoxidation inhibitor can be promoted if, in the presence of the oxidationinhibitor, mechanical energy is applied to the silver-based powder bysubjecting it to crushing, shocking, rolling, etc.

The following raw materials can be used for the preparation of thesilver-based powder of the invention: a reduced silver powder preparedin a granulated form by reducing an aqueous solution of a silver nitratewith the use of such reducing agents as hydrazine, formaldehyde,ascorbic acid, etc.; an electrolytic silver powder precipitated in adendrite form on a cathode by subjecting an aqueous solution of silvernitrate to electrolytic decomposition; or atomized silver particlesobtained in a granulated or irregular form by pulverizing molten silverhot-melted at a temperature above 1000° C. into water or inert gas. Thepowders may comprise fine powders of pure silver, silver-copper alloy,silver-palladium alloy or alloys of silver with other metals, such aszinc, tin, magnesium, nickel, etc.

Furthermore, the oxidation inhibitors for treating the surface of thesilver-based powder of the invention may comprise a phenol-basedcompound, hindered phenol-based compounds and triazole-based compounds.The following are examples of the phenol-based compounds:2,6-di-t-butyl-4-methyl phenol and 2,2′-methylene-bis(6-t-butyl-4-methylphenol). The following are examples of the hinderedphenol-based compounds:triethyleneglycol-bis[3-(3-t-butyl-5-methyl-4-hydroxyphenyl)propionate,2,4-bis(n-octylthio)-6-(4-hydroxy-3,5-di-t-butylanilino)-1,3,5-triazine,and 3,5-di-t-butyl-4-hydroxybenzyl-phosphonatediethyl ester. Thetriazole-based compounds can be exemplified by triazole, benzotriazole,4-methylbenzotriazole, 5-methylbenzotriazole,2-(5-methyl-2-hydroxyphenyl)benzotriazole,2-(3,5-di-t-butyl-2-hydroxyphenyl)benzotriazole, and2-(2′-hydroxy-5′-t-octylphenyl)benzotriazole. The benzotriazole-basedcompounds are preferred. The oxidation inhibitors may be used in acombination of two or more.

The silver-based powder particles suitable for application to them ofmechanical energy may have the shape of flakes, dendrite flakes, or mayhave an irregular shape. Although there are no special restrictions withregard to the average size of the particles, it is recommended that theyhave the average size within the range of 1 to 20 micrometers (μm).

Since the silver-based powder of the invention possesses excellentelectrical conductivity and thermal conductivity, it is suitable for useas an electrically conductive filler and a thermally conductive fillerfor compounding with thermosetting organic resins, thermoplastic organicresins, or curable silicone compositions. In particular, the mostsignificant decrease in the change of curability and electricalproperties of curable silicone compounds during storage is noticed whenthese compounds are combined with the silver-based powder of theinvention. Therefore, the most advantageous application of thesilver-based powder of the invention is an electrically conductivefiller and a thermally conductive filler for curable siliconecompositions.

The preparation method of this invention is characterized by using anorganic solution of an oxidation inhibitor as a lubricating agent andapplying a mechanical energy to the silver-based powder. Thesilver-based powder suitable for use in the method of the presentinvention may comprise the aforementioned reduced silver powder,electrolytic silver powder, or the atomized silver powder. Thesilver-based powder may be prepared from pure silver, or from asilver-copper alloy, silver-palladium alloy, or alloys of silver withminute quantities of other metals such as zinc, tin, magnesium, nickel ,etc. Although there are no special restrictions with regard to the sizeof the particles in the powder, to obtain the average size ofsilver-based particles within the range of 0.1 to 20 μm, it isrecommended that a raw silver-based powder (used to prepare thesilver-based powder of this invention) has an average particle sizewithin the range of 0.1 to 50 μm. There are no restrictions on the shapeof the silver-based particles, and the particles may have granular,dendrite, flake-like, or irregular shapes. Particles of two or moredifferent shapes can be used in a mixture.

A majority of the aforementioned oxidation inhibitors are solidsubstances, so in the method of this invention the oxidation inhibitorsmay be used in the form of organic solutions. Organic solvents suitablefor preparation of the organic solutions may be represented by methanol,ethanol, isopropanol, or similar alcoholic solvents; hexane, heptane,octane, or similar aliphatic solvents; cyclohexane, cyclooctane, orsimilar alicyclic solvents; toluene, xylene, or similar aromaticsolvents; acetone, methylethylketone, methyl isobutylketone, or similarketone-type solvents; and ethyl acetate, carbitol acetate, or similarester-type solvents.

There are no special restrictions on the amounts in which the oxidationinhibitor should be added. However, it is recommended to add theoxidation inhibitor in an amount of 0.01 to 2 parts by weight per 100parts by weight of the silver-based powder. If the oxidation inhibitoris added in an amount below the lower recommended limit of the range, itwould be impossible to provide sufficient surface treatment of thesilver powder. If the upper recommended limit is exceeded, the obtainedsilver-based powder either will have a reduced electrical and thermalconductivity, or lower affinity to the curable organic resincomposition.

In accordance with the method of the invention, a mechanical energy isapplied to the composition comprising raw silver-based powder andoxidation inhibitor. A mechanical reaction will activate the surface ofthe raw silver-based powder and promote the chemical reaction in whichthe oxidation inhibitor participates. In application of mechanicalenergy to the raw silver-based powder, the silver-based powder issurface-treated with the oxidation inhibitor, and the organic solventsolution of the oxidation inhibitor functions as a lubrication agent.

The mechanical energy can be applied by using stamp mill, ball mill,vibration mill, hammer mill, rolling mill, mortar, or a similar crusher,etc. There are no special restrictions with regard to the treatmenttemperature, but since the treatment is accompanied with generation ofheat, the temperature should be adjusted between room temperature and100° C. Treatment can be carried out from 1 to 50 hours. Since thetreated silver-based powder can be coated with an excessive amount ofthe oxidation inhibitor stuck to the surface of the silver-basedparticles, if necessary, the excess of the inhibitor can be removed byrinsing the treated powder in an organic solvent with subsequent dryingfor 24 hours or more at a temperature from room temperature to 105° C.

The curable silicone composition of the invention comprises theaforementioned silver-based powder surface-treated with an oxidationinhibitor. In compounding with a curable silicone composition, theflake-like shape is preferred for obtaining cured silicone bodies ofhigh electrical conductivity.

It is preferable to treat the surface of the silver-based powder withthe oxidation inhibitor uniformly, but partial treating of the surfaceis also allowable. Although it is preferable to provide chemical bondsbetween the oxidation inhibitor and the silver-based powder surface,mere adherence to the surface is also allowable. When a coating film ofthe oxidation inhibitor is formed on the surface of the silver-basedpowder particles, the thinner the thickness of the coating film, thebetter the electrical and chemical conductivity of the cured siliconeobtained from a mixture of the silver-based powder with the curablesilicone composition. However, since curability of the curable siliconecomposition noticeably varies over time during storage, the thickness ofthe coating film should be optimally selected. Preferably, it should bethinner than 0.1 μm. The surface of the silver-based powder particlesmay appear to be treated with an excess of the oxidation inhibitor thatadheres to the surface but does not participate in surface treatment. Ashas been explained above, this excess can be removed by rinsing in anorganic solvent.

In preparation of the silver-based powder by the method of theinvention, mechanical energy can be applied to the raw silver-basedpowder either after soaking with an organic solution of the oxidationinhibitor, or when the raw silver-based is immersed into theaforementioned organic solution. For soaking, the raw silver-basedpowder is loaded into the organic solution of the oxidation inhibitor,and, if necessary, is stirred.

There is no special restriction with regard to the treatmenttemperature, but the treatment temperature is preferably between roomtemperature and 100° C. It is preferable to conduct the immersion for 1hour to 50 hours. The silver-based powder is then removed from thesolution, if necessary, rinsed in an organic solvent for removal of theexcess of the oxidation inhibitor adhered to the surface of theparticles, and dried for more than 24 hours at a temperature betweenroom temperature and 105° C.

In an alternative, second process, while using the organic solution ofthe oxidation inhibitor as a lubrication agent, the raw silver-basedpowder is subjected to application of mechanical energy in aconventional powder crushing device such as a stamp mill, ball mill,vibration mill, hammer mill, rolling mill, mortar, or the like. There isno special restriction with regard to the treatment temperature in thisprocess, but since the process is accompanied by generation of heat, thetemperature should be adjusted to a value between room temperature and100° C., and the process should be conducted for 1 hour to 50 hours. Ifit is necessary to remove the excess of the oxidation inhibitor stuck tothe surface of the particles, the treated silver-based powder may berinsed in an organic solvent and dried for more than 24 hours at atemperature between room temperature and 105° C. Such a process makes itpossible to produce flake-like silver-based particles that possesselectrical and thermal conductivity. Furthermore, crushing of thesilver-based powder activates the surface of the silver particles,improves adherence and chemical bonding of the oxidation inhibitor tothis surface, prevents aggregation of the particles, and promotesformation of the flaky shape.

There are no special restriction with regard to the size of the silverparticles used in the second method, but in order to obtain particleshaving an average size within the range of 0.1 to 20 μm, it ispreferable that a raw silver-based powder (used to prepare thesilver-based powder of this invention) has an average particle sizewithin the range of 0.1 to 50 μm. There are no restrictions on the shapeof the silver particles, and the particles may have granular, dendrite,flake-like, or irregular shapes. Particles of two or more differentshapes can be used in a mixture.

There are no special restrictions with regard to the amounts in whichthe oxidation inhibitor should be added. However, it is recommended toadd oxidation inhibitor in an amount of 0.01 to 2 parts by weight per100 parts by weight of the silver-based powder. If the oxidationinhibitor is added in an amount below the lower recommended limit of therange, it would be impossible to provide sufficient surface treatment ofthe silver powder. If the upper recommended limit is exceeded, theobtained silver powder will have either a reduced electrical and thermalconductivity, or lower affinity to the curable organic resincomposition.

There are no special restrictions on the mechanism for curing thecurable silicone composition of this invention. For example, curing canbe carried out by means of a hydrosilylation reaction, condensationreaction, or free-radical reaction with the use of organic peroxide. Theuse of a silicone composition curable with the hydrosilylation reactionis preferable. Such silicone composition may comprise the followingcomponents:

(A) 100 parts by weight of an organopolysiloxane having at least twoalkenyl groups per molecule;

(B) an organopolysiloxane having at least two silicon-bonded hydrogenatoms per molecule, where the silicon-bonded hydrogen atoms of component(B) are used in an amount of 0.5 to 5 per one alkenyl groups ofcomponent (A);

(C) 50 to 2,000 parts by weight of a silver-based powder surface-treatedwith an oxidation inhibitor; and

(D) a platinum catalyst in an amount required for promoting thehydrosilylation reaction.

Component (A) is an organopolysiloxane with at least two alkenyl groupsper molecule. The alkenyl groups of component (A) may be represented byvinyl groups, allyl groups, butenyl groups, pentenyl groups, hexenylgroups, and heptenyl groups, of which the vinyl groups are preferable.There is no special restriction with regard to the bonding positions ofthe aforementioned alkenyl groups and they may assume positions on themolecular terminals and/or in the side chains. In component (A),silicon-bonded organic groups, other than the aforementioned alkenylgroups, may comprise substituted or unsubstituted univalent hydrocarbongroups, such as methyl groups, ethyl groups, propyl groups, butylgroups, pentyl groups, hexyl groups, or similar alkyl groups; phenylgroups, tolyl groups, xylyl groups, or similar aryl groups; benzylgroups, phenethyl groups, or similar aralkyl groups;3,3,3-trifluoropropyl groups, or similar halogenated alkyl groups.Methyl groups and phenyl groups are preferred. There are no restrictionson the molecular structure of component (A). Component (A) may have alinear structure, partially-branched linear structure, branchedstructure, or net-like structure, of which the linear structure, or apartially branched linear structure are preferable. There are norestrictions on viscosity of component (A) at 25° C., but preferably,viscosity should be between 50 and 500,000 milliPascal.seconds (mPa.s),and especially between 400 and 10,000 mPa.s.

Examples of organopolysiloxanes of component (A) include a copolymer ofmethylvinylsiloxane and dimethylsiloxane having both terminal ends ofthe molecular chain blocked by trimethylsiloxy groups, amethylvinylpolysiloxane having both terminal ends of the molecular chainblocked by trimethylsiloxy groups, a copolymer of methylphenylsiloxaneand methylvinylsiloxane having both terminal ends of the molecular chainblocked by trimethylsiloxy groups, a copolymer of methylphenylsiloxane,methylvinylsiloxane, and dimethylsiloxane having both terminal ends ofthe molecular chain blocked by trimethylsiloxy groups, adimethylpolysiloxane having both terminal ends of the molecular chainblocked by dimethylvinylsiloxy groups, a methylvinylpolysiloxane havingboth terminal ends of the molecular chain blocked by dimethylvinylsiloxygroups, a methylphenylpolysiloxane having both terminal ends of themolecular chain blocked by dimethylvinylsiloxy groups, a copolymer ofmethylvinylsiloxane and dimethylsiloxane having both terminal ends ofthe molecular chain blocked by dimethylvinylsiloxy groups, a copolymerof methylphenylsiloxane and dimethylsiloxane having both terminal endsof the molecular chain blocked by dimethylvinylsiloxy groups, acopolymer of methylvinylsiloxane and dimethylsiloxane having bothterminal ends of the molecular chain blocked by silanol groups, amethylvinylpolysiloxane having both terminal ends of the molecular chainblocked by silanol groups, a copolymer of methylphenylsiloxane,methylvinylsiloxane, and dimethylsiloxane having both terminal ends ofthe molecular chain blocked by silanol groups, silicone resins composedof R₃SiO_(1/2) units and SiO_(4/2) units, silicone resins composed ofRSiO_(3/2) units, silicone resins composed of R₂SiO_(2/2) units andRSiO_(3/2) units, silicone resins composed of R₂SiO_(2/2) units,RSiO_(3/2) units, and SiO_(4/2) units, or mixtures of the aforementionedcompounds. In the resins, each R designates substituted or unsubstitutedunivalent hydrocarbon group, and at least two R's in one molecule ofeach of the aforementioned resins are alkenyl groups. In the resins, Rmay be the same substituted or unsubstituted alkenyl groups or thosegroups other than alkenyl groups that have been mentioned above.

Component (B) is used as a cross-linking agent for curing component (A)and comprises an organopolysiloxane that has at least two silicon-bondedhydrogen atoms per molecule. There are no special restriction withregard to the bonding positions of the hydrogen atoms in component (B),and they may assume positions on the molecular terminals and/or in theside chains. In component (B), silicon-bonded organic groups, other thanthe aforementioned hydrogen atoms, may comprise substituted orunsubstituted univalent hydrocarbon groups, preferably, methyl groupsand phenyl groups. There are no restrictions on the molecular structureof component (B). Component (B) may have a linear structure,partially-branched linear structure, branched structure, or net-likestructure, of which the linear structure, or a partially branched linearstructure are preferable. There are no restrictions on viscosity ofcomponent (B) at 25° C., but preferably, it should be between 1 and50,000 mPa.s, and especially between 5 and 1,000 mPa.s.

Examples of organopolysiloxanes of component (B) include amethylhydrogensiloxane having both terminal ends of the molecular chainblocked by trimethylsiloxy groups, a copolymer of methylhydrogensiloxaneand dimethylsiloxane having both terminal ends of the molecular chainblocked by trimethylsiloxy groups, a copolymer of methylphenylsiloxaneand methylhydrogensiloxane having both terminal ends of the molecularchain blocked by trimethylsiloxy groups, a copolymer ofmethylphenylsiloxane, methylhydrogensiloxane, and dimethylsiloxanehaving both terminal ends of the molecular chain blocked bytrimethylsiloxy groups, a dimethylpolysiloxane having both terminal endsof the molecular chain blocked by dimethylhydrogensiloxy groups, amethylhydrogenpolysiloxane having both terminal ends of the molecularchain blocked by dimethylhydrogenpolysiloxy groups, a copolymer ofmethylhydrogensiloxane and dimethylsiloxane having both terminal ends ofthe molecular chain blocked by dimethylhydrogensiloxy, a copolymer ofmethylphenylsiloxane and dimethylsiloxane having both terminal ends ofthe molecular chain blocked by dimethylhydrogensiloxy groups, amethylphenylpolysiloxane having both terminal ends of the molecularchain blocked by dimethylhydrogensiloxy groups, amethylhydrogenpolysiloxane having both terminal ends of the molecularchain blocked by silanol groups, a copolymer of methylhydrogensiloxaneand dimethylsiloxane having both terminal ends of the molecular chainblocked by silanol groups, a copolymer of methylphenylsiloxane andmethylhydrogensiloxane having both terminal ends of the molecular chainblocked by silanol groups, and a copolymer of methylphenylsiloxane,methylhydrogensiloxane, and dimethylsiloxane having both terminal endsof the molecular chain blocked by silanol groups.

Component (B) should be used in an amount sufficient to provide 0.5 to 5of silicon-bonded hydrogen atoms contained in component (B) for 1alkenyl group of component (A). If component (B) is used in an amountbelow the lower recommended limit of the range, it would be impossibleto provide sufficient curing of the obtained composition. If the upperrecommended limit is exceeded, the obtained cured body of silicone willhave reduced thermal resistance.

Component (C) is the aforementioned silver-based powder. Component (C)imparts to a cured body obtained by curing the composition of theinvention electroconductive and thermoconductive properties. Component(C) should be used in an amount of 50 to 2000 parts by weight,preferably 300 to 600 parts by weight for each 100 parts by weight ofcomponent (A). If component (C) is used in an amount below the lowerrecommended limit of the range, the obtained cured silicone body willhave low electrical conductivity. If the upper recommended limit isexceeded, the obtained composition will be difficult to handle and moldin the production.

Component (D) is a platinum-type catalyst that accelerates curing of thepresent composition. Normally, this is a known platinum-type catalystused in hydrosilylation reactions. Component (D) can be represented byplatinum black, platinum on an alumina carrier, platinum on a silicacarrier, platinum on a carbon carrier, a chloroplatinic acid, analcoholic solution of a chloroplatinic acid, a platinum-olefin complex,a complex of platinum and alkenyl siloxane, or catalysts formed bydispersing the aforementioned compounds in thermoplastic organic resinssuch as methylmethacrylate, polycarbonate, polystyrene, silicone resin,etc.

Although there are no restrictions with regard to the amounts in whichcomponent (D) can be used in the composition of the invention, it isrecommended that in terms of weight units metallic platinum be withinthe range of 0.1 to 1000 parts per million (ppm). If component (D) isadded in an amount below the lower recommended limit of the range, theobtained curable silicone composition will be insufficiently cured. Ifthe upper recommended limit is exceeded, the obtained cured product mayencounter coloration problems.

The composition is prepared by uniformly mixing components (A) through(D). To minimize variations in contact resistance and volumetricresistivity of the cured body over time, the composition may beadditionally combined with component (E), an organosilicon compound thatcontains silicon-bonded alkoxy groups. The following are examples ofsuch organosilicon compounds: tetramethoxysilane, tetraethoxysilane,dimethyldimethoxysilane, methylphenyldimethoxysilane,methylphenyldiethoxysilane, phenyltrimethoxysilane,methyltrimethoxysilane, methyltriethoxysilane, vinyltrimethoxysilane,allyltrimethoxysilane, allyltriethoxysilane,3-glycidoxypropyltrimethoxysilane, 3-methacryloxypropyltrimethoxysilane,or similar alkoxysilanes; siloxanes represented by the followingformulae:

where “a” and “b” are integers that are equal to 1 or exceed 1.

In the composition of the invention, component (E) is optional. Whencomponent (E) is included, it should be used in an amount of less than20 parts by weight, preferably 0.5 to 10 parts by weight for each 100parts by weight of component (A). Without the use of component (E), thecurable silicone composition may either have poor adhesiveness orelectrical resistance and volumetric resistivity changeable over time inthe cured product. If component (E) is used in an amount exceeding theupper recommended limit of the range, either the obtained curablesilicone composition will have low storage stability, or the curedproduct obtained from the silicone composition will have physicalproperties changeable over time.

To improve storage stability and facilitate handling during treatment,the composition of the invention can be additionally combined with otheradditives, such acetylenic alcohols exemplified by2-methyl-3-butyn-2-ol, 3,5-dimethyl-1-hexyn-3-ol, 2-phenyl-3-butyn-2-ol,or similar alkyne alcohols; 3-methyl-3-penten-1-yne;3,5-dimethyl-3-hexen-1-yne, or similar enyne compounds;1,3,5,7-tetramethyl-1,3,5,7-tetravinylcyclotetrasiloxane,1,3,5,7-tetramethyl-1,3,5,7-tetrahexenylcyclotetrasiloxane, or a similaralkenylsiloxane oligomer; benzotriazole, or similar curing inhibitors.Although there are no special restriction with regard to the amounts inwhich the curing inhibitors can be added, it is recommended to add themin an amount of 0.001 to 5 parts by weight for each 100 parts by weightof component (A).

To impart to the cured product an increased hardness and strength, thecomposition can be combined with an inorganic filler, such as fumedsilica, crystalline silica, baked silica, wet-process silica, fumedtitanium oxides, etc.; or inorganic fillers surface treated withorganosilicon compounds, such as organoalkoxysilane, organochlorosilane,organodisilazane, etc. Although there are no special restrictions withregard to the amounts in which the aforementioned inorganic fillers canbe added, it is recommended to add them in an amount of 50 parts byweight or less for each 100 parts by weight of component (A).

There are no special restrictions on the form in which the cured productcan be obtained. For example, it can be formed as a rubber, gel, or asimilar elastomer, or as a hard resin. The elastomeric form ispreferable. Especially when component (C) is a flake-like silver powder,the cured product obtained from the composition of the invention mayhave volumetric electrical resistivity of 0.1 Ohm.cm or less, and even1×10⁻³ Ohm.cm or less, and thermal conductivity of 1 W/mK or more, andeven 3 W/mK or more. The composition of the invention may be used as araw material for manufacturing electroconductive adhesive agents,heat-radiating adhesive agents, electroconductive die-bonding agents,heat-radiating die-bonding agents, electroconductive pastes,heat-radiating pastes, electromagnetic shielding agents, as well as formanufacturing electroconductive sheets, heat-radiating sheets, andelectromagnetic-wave absorption sheets.

EXAMPLES

In these examples, all viscosities correspond to 25° C. Evaluations ofchanges over time in such properties as curability of the composition,volumetric resistivity of the cured product, thermal conductivity, andadhesive properties were carried out as described below.

Change in Curability of the Curable Silicone Composition over Time

After the curable silicone composition was prepared and cooled, it wascured by heating for 30 minutes (min.) at 150° C. directly after thepreparation (initial stage), after one-week storage, after one-monthstorage, and after three-month storage, and then hardness of the curedbody of each type was measured with the use of a Type A durometer asspecified by JIS K 6253.

Change in the Volumetric Resistivity of the Silicone Composition overTime

The curable silicone composition was heat-treated for 30 min. at 150° C.and formed into a cured silicone sheet having a thickness exceeding 1millimeter (mm). Volumetric resistivity of the sheet was measured usinga resistivity measurement device (model K-705RL manufactured by KyowaRiken Ltd.). For evaluating changes in the volumetric resistivity of thesilicone sheet over time, the sheet was heated in an oven at 150° C.,and the volumetric resistivity was measured by the method mentionedabove after heating the sheet during 100 hours, 500 hours, and 1000hours.

Thermal Conductivity of the Cured Silicone Product

A 1 mm-thick cured silicone sheet was made by heating the siliconecomposition of the invention at 150° C. for 30 min. Thermal conductivityof the sheet was calculated on the basis of specific heat, density, anda coefficient of thermal diffusion obtained with the use of a laserflash instrument.

Adhesiveness of the Cured Silicone Product

The curable silicone composition was spread over a nickel substrate andthen formed by heating for 30 min. at 150° C. into a cured siliconecoating. In evaluating conditions of peeling of the silicone coatingfrom the nickel substrate, peeling with rupture of the cured siliconewas designated as o, and peeling with separation along the interface wasdesignated as X.

Change in Curability of a Curable Epoxy Composition over Time

After a curable epoxy composition was prepared and cooled, itsappearance was visually observed directly after the preparation (initialstage), after one-week storage, after one-month storage, and afterthree-month storage, and then curability was evaluated by curing thecomposition at 150° C. for 1 hour.

Change in Volumetric Resistivity of the Cured Epoxy Resin Product overTime

The curable epoxy composition was heat-treated for 1 hour at 150° C. andformed into a cured epoxy sheet having a thickness exceeding 1 mm.Volumetric resistivity of the sheet was measured using a resistivitymeasurement device (model K-705RL manufactured by Kyowa Riken Ltd.). Forevaluating changes in the volumetric resistivity of the epoxy sheet overtime, the sheet was heated in an oven at 150° C., and the volumetricresistivity was measured by the method mentioned above after heating thesheet during 100 hours, 500 hours, and 1000 hours.

Thermal Conductivity of the Cured Epoxy Product

A 1 mm-thick cured epoxy sheet was made by heating the curable epoxycomposition at 150° C. for 1 hour. Thermal conductivity of the sheet wascalculated on the basis of specific heat, density, and a coefficient ofthermal diffusion obtained with the use of a laser flash instrument.

Adhesiveness of the Cured Epoxy Product

The curable epoxy composition was spread over a nickel substrate andthen formed into a cured epoxy coating by heating for 1 hour at 150° C.In evaluating conditions of peeling of the epoxy coating from the nickelsubstrate, peeling with rupture of the cured epoxy and with theformation of cracks in the cured coating was designated as o, andpeeling with separation along the interface was designated as X.

Example 1

20 grams (g) of silver nitrate were dissolved in 40 milliliters (ml) ofwater, and then a 46 wt. % aqueous solution of sodium hydroxide wasadded to cause precipitation of granular-shaped silver oxide. Aftersubjecting the obtained granular-shaped silver oxide to reduction informalin, the product was rinsed, filtered several times, and dried atroom temperature. A reduced silver powder having granular-shapedparticles with an average particle size of 2 μm was prepared.

200 g of the reduced silver powder prepared by repeating the method asmentioned above and 80 g of a 30 wt. % acetone solution of benzotriazoleas a lubricant for the process were loaded into a ball mill, where thereduced silver powder was crushed. After the crushed silver powder waswashed with acetone and dried at room temperature, silver powder (I)surface-treated with benzotriazole was prepared.

A uniform mixture was prepared from 100 parts by weight of a 10 Pa.sviscosity dimethylpolysiloxane having both molecular terminals cappedwith dimethylvinylsiloxy groups, 0.71 parts by weight of a 10 mPa.sviscosity methylhydrogenpolysiloxane having both molecular terminalscapped with trimethylsiloxy groups (this component contains twosilicon-bonded hydrogen atoms per 1 vinyl group in the aforementioneddimethylpolysiloxane), 1035 parts by weight of the silver powdersurface-treated as mentioned above, 23 parts by weight of a siloxanecompound represented by the following formula:

a fine-powdered platinum catalyst (added, in weight units, in an amountof 32 ppm of the metallic platinum relative to the weight of thecomposition), and 2-phenyl-3-buten-2-ol (added, in weight units, in anamount of 60 ppm relative to the weight of the composition). Thus, ahydrosilylation-curable silicone-rubber composition was prepared.

The obtained silicone rubber composition was evaluated with regard tochange of curability and volumetric resistivity over time, as well aswith regard to thermal conductivity and adhesiveness in the curedsilicone rubber product obtained from the aforementioned composition.The results are shown in Table 1.

Example 2

200 g of atomized silver powder having spherical particles of a 5 μmaverage particle size and produced by atomization in water were immersedfor 5 hours at 50° C. into 80 g of a 10 wt. % acetone solution ofbenzotriazole. The solution was filtered and dried at room temperature.As a result, silver powder (II) surface treated with benzotriazole wasproduced.

A hydrosilylation-curable silicone rubber composition was prepared bythe same method as in Example 1, with the exception that the silverpowder (II) surface treated with benzotriazole was used instead of thesilver powder of Example 1. The obtained silicone rubber composition wasevaluated with regard to change of curability and volumetric resistivityover time, as well as with regard to thermal conductivity andadhesiveness in the cured silicone rubber product obtained from theaforementioned composition. The results are shown in Table 1.

Comparative Example 1

20 g of silver nitrate were dissolved in water, and then a 46 wt. %aqueous solution of sodium hydroxide were added to cause precipitationof granular-shaped silver oxide. After subjecting the obtainedgranular-shaped silver oxide to reduction in formalin, the product wasrinsed, several times filtered, and dried at room temperature. A reducedsilver powder having granular-shaped particles with an average particlediameter of 2 μm was prepared.

200 g of the reduced silver powder prepared by repeating the method asmentioned above and 100 g of a 20 wt. % carbitol acetate solution ofstearic acid as a lubricant for the process were loaded into a ballmill, where the reduced silver powder was crushed. After the crushedsilver powder was washed with methanol and dried at room temperature,silver powder (III) surface-treated with stearic acid was prepared.

A hydrosilylation-curable silicone rubber composition was prepared bythe same method as in Example 1, with the exception that theaforementioned silver powder (III) was used instead of the silver powderof Example 1. The obtained silicone rubber composition was evaluatedwith regard to change of curability and volumetric resistivity overtime, as well as with regard to thermal conductivity and adhesiveness inthe cured silicone rubber product obtained from the aforementionedcomposition. The results are shown in Table 1.

Comparative Example 2

A hydrosilylation-curable silicone rubber composition was prepared bythe same method as in Example 1, with the exception that an atomizedsilver powder (IV) with an average particle diameter of 5 μm was usedinstead of the silver powder of Example 1. The obtained silicone rubbercomposition was evaluated with regard to change of curability andvolumetric resistivity over time, as well as with regard to thermalconductivity and adhesiveness. The results are shown in Table 1.

Example 3

200 g of atomized silver powder for use as a lubricating agent havingspherical particles of a 2 μm average particle size and produced byatomization in water were immersed into 80 g of a 30 wt. % acetonesolution of benzotriazole, and the atomized sliver powder was thencrushed in a ball mill. The crushed silver powder was washed withacetone and dried at room temperature. As a result, silver powder (V)surface treated with benzotriazole was produced.

A hydrosilylation-curable silicone rubber composition was prepared bythe same method as in Example 1, with the exception that silver powder(V) was used instead of the silver powder of Example 1. The obtainedsilicone rubber composition was evaluated with regard to change ofcurability and volumetric resistivity over time, as well as with regardto thermal conductivity and adhesiveness in the cured silicone rubberproduct obtained from the aforementioned composition. The results areshown in Table 1.

Comparative Example 4

200 g of atomized silver powder having spherical particles of a 2 μmaverage particle size and a lubricating agent in the form of 100 g of a2000 mPa.s viscosity 10 wt. % xylene solution of dimethylpolysiloxanehaving both molecular terminals capped with dimethylvinylsiloxy groupswere loaded into a ball mill, where the atomized silver powder wascrushed. The crushed silver powder was washed with xylene and dried for24 hours at 150° C. As a result, silver powder (VI) surface treated withdimethylpolysiloxane was produced.

A hydrosilylation-curable silicone rubber composition was prepared bythe same method as in Example 1, with the exception that silver powder(VI) was used instead of the silver powder of Example 1. The obtainedsilicone rubber composition was evaluated with regard to change ofcurability and volumetric resistivity over time, as well as with regardto thermal conductivity and adhesiveness in the cured rubber product.The results are shown in Table 1.

Practical Example 4

200 g of electrolytic having a dendrite shape and a 10 μm averageparticle size and a lubricating agent in the form of 80 g of 10 wt. %acetone solution of 2,6-di-t-butyl-4-methylphenol were loaded into aball mill, where the electrolytic silver powder was crushed. The crushedsilver powder was washed with acetone and dried at room temperature. Asa result, silver powder (VII) surface treated with benzotriazole wasproduced.

A hydrosilylation-curable silicone rubber composition was prepared bythe same method as in Example 1, with the exception that theaforementioned silver powder (VII) was used instead of the silver powderof Example 1. The obtained silicone rubber composition was evaluatedwith regard to change of curability and volumetric resistivity overtime, as well as with regard to thermal conductivity and adhesiveness inthe cured silicone rubber product obtained from the aforementionedcomposition. The results are shown in Table 1. TABLE 1 CategoriesPresent Invention Comparative Examples Characteristics Example 1 Example2 Example 3 Example 4 Comp. Ex. 1 Comp. Ex. 2 Comp. Ex. 3 Untreatedsilver powder Reduced Atomized Atomized Electrolytic Reduced AtomizedAtomized silver silver silver silver silver silver silverSurface-treating agent Benzotriazole Benzotriazole Benzotriazole2,6-di-t- Stearic acid None Dimethylpolysiloxane butyl-4- methylphenolSilver powder after surface treatment Average particle size (μm)  7  5 510  7  5 5 Shape Flake shape Spherical Flake shape Flake shape Flakeshape Flake shape Flake shape Properties of Silicone Rubber HardnessInitial 60 52 50 60 60 50 50 After 1 day storage 60 52 50 60 55  5 10After 1 week storage 60 52 50 60 40 Uncured Uncured After 1 monthstorage 60 52 50 60 20 Uncured Uncured After 3 month storage 58 52 50 59Uncured Uncured Uncured Volumetric resistivity (Ohm · cm) Initial 3 ×10⁻⁴ 100  0.1 3 × 10⁻⁴ 3 × 10⁻⁴ 100  0.1 After 100 hours 3 × 10⁻⁴ 100 0.1 3 × 10⁻⁴ 6 × 10⁻⁴ 100  0.1 After 500 hours 3 × 10⁻⁴ 100  0.1 3 ×10⁻⁴ 2 × 10⁻³ 100  0.1 After 1000 hours 4 × 10⁻⁴ 100  0.1 5 × 10⁻⁴ 3 ×10⁻³ 100  0.1 Thermal conductivity  4   0.8 1  3  4   0.8  1 (° C./W)Adhesiveness ◯ ◯ ◯ ◯ X ◯ ◯

Example 5

A curable epoxy resin composition was prepared by adding silver powder(V) of Example 3 in an amount of 75 wt. % to a commercial curable epoxyresin (EP-106 of Cemedine Co.). The obtained epoxy rubber compositionwas evaluated with regard to change of curability and volumetricresistivity over time, as well as with regard to thermal conductivityand adhesiveness. The results are shown in Table 2.

Comparative Example 4

A curable epoxy resin composition was prepared by the same method as inExample 5, with the exception that the aforementioned silver powder (II)was used instead of silver powder (V) of Example 3. The obtained epoxyresin composition was evaluated with regard to change of curability andvolumetric resistivity over time, as well as with regard to thermalconductivity and adhesiveness in the cured epoxy resin product obtainedfrom the composition. The results are shown in Table 2. TABLE 2Categories Present Invention Comparative Example Characteristics Example5 Comp. Ex. 4 Untreated silver powder Atomized silver Atomized silverSurface-treating agent Benzotriazole Benzotriazole Silver powder aftersurface treatment Average particle size (μm) 5 5 Shape Flake shapeSpherical Properties of Composition Curability and appearance InitialCured Cured After 1 day storage Cured Cured After 1 week storage CuredGelation After 1 month storage Cured Gelation After 3 month storageCured Gelation Properties of cured Product Volumetric resistivity (Ohm ·cm) Initial 3 × 10⁻⁴ 2 × 10⁻² After 100 hours 3 × 10⁻⁴ 2 × 10⁻² After500 hours 3 × 10⁻⁴ 2 × 10⁻² After 1000 hours 4 × 10⁻⁴ 2 × 10⁻² Thermalconductivity 4 2 (° C./W) Adhesiveness ∘ ∘

INDUSTRIAL APPLICABILITY

When a curable organic resin composition is compounded with thesilver-based powder of the present invention, the composition becomesless prone to a decrease in curability over time. Furthermore, themethod of preparation of the silver-based powder in accordance with theinvention is highly efficient. Another effect of the invention is that acured product obtained from the aforementioned composition ischaracterized by reduced change in curability and electrical propertiesover time.

1. A silver-based powder characterized by being surface-treated with anoxidation inhibitor by means of a mechanochemical reaction.
 2. Thepowder of claim 1, where the oxidation inhibitor is a phenol-basedcompound, hindered phenol-based compound, or triazole-based compound. 3.A method of preparation of a silver-based powder, said method comprisingthe steps of: a) utilizing an organic solution of an oxidation inhibitoras a lubricating agent, b) applying mechanical energy to thesilver-based powder, and c) subjecting the silver-based powder tosurface treatment with the oxidation inhibitor.
 4. The method of claim3, where the oxidation inhibitor is a phenol-based compound, hinderedphenol-based compound, or triazole-based compound.
 5. A compositioncomprising a curable silicone composition and a silver-based powdersurface-treated with an oxidation inhibitor.
 6. The composition of claim5, where the silver-based powder is surface-treated with the oxidationinhibitor by a mechanochemical reaction.
 7. The composition of claim 5,where the oxidation inhibitor is a phenol-based compound, hinderedphenol-based compound, or triazole-based compound.
 8. The composition ofclaim 5, where the curable silicone composition is curable with ahydrosilylation reaction.
 9. The composition of claim 8, comprising: (A)100 parts by weight of an organopolysiloxane having at least two alkenylgroups per molecule; (B) an organopolysiloxane having at least twosilicon-bonded hydrogen atoms per molecule, where component (B) ispresent in an amount sufficient to provide silicon-bonded hydrogen atomsin an amount of 0.5 to 5 per one alkenyl group of component (A); (C) 50to 2,000 parts by weight of the silver-based powder, surface-treatedwith the oxidation inhibitor, for each 100 parts by weight of component(A); and (D) a platinum catalyst in an amount required for promoting thehydrosilylation reaction.
 10. Use of the composition of claim 5 as anelectroconductive adhesive agent, heat-radiating adhesive agent,electroconductive die-bonding agent, heat-radiating die-bonding agent,electroconductive paste, heat-radiating paste, electromagnetic shieldingagent, or raw material for manufacturing an electroconductive sheet,heat-radiating sheet, or electromagnetic-wave absorption sheet.
 11. Thepowder of claim 1, where the mechanochemical reaction includes applyingmechanical energy to the powder to activate a surface of the powder andreacting the oxidation inhibitor with the activated surface of thepowder.
 12. The powder of claim 11, where the applying of the mechanicalenergy to the powder includes crushing, shocking, or rolling the powder.13. The powder of claim 1, where the oxidation initiator is present inan amount of 0.01 to 2 parts by weight per 100 parts by weight of thepowder.
 14. The method of claim 3, where applying mechanical energy tothe silver-based powder includes crushing, shocking, or rolling thepowder.
 15. The method of claim 3, where the organic solution comprisesthe oxidation inhibitor and an organic solvent selected from the groupof alcoholic solvents, aliphatic solvents, aromatic solvents, ester-typesolvents, and combinations thereof.
 16. The method of claim 3, where theoxidation initiator is present in an amount of 0.01 to 2 parts by weightper 100 parts by weight of the silver-based powder.
 17. The compositionof claim 5, where the oxidation initiator is present in an amount of0.01 to 2 parts by weight per 100 parts by weight of the silver-basedpowder.
 18. The composition of claim 9, where the silver-based powder,surface-treated with the oxidation inhibitor, is present in an amount of300 to 600 parts by weight for each 100 parts by weight of component(A).